Full bars means 75-100% power remaining.
Very roughly yeah, but not really. What you don't want to do is assume that, say, if there are four bars that each bar correlates to 1/4 of the battery energy.
Look at the discharge curve of any battery and you'll understand why. The curve shows how the voltage decreases over time. The number of bars is actually an indication of the battery voltage. Although I suppose a manufacturer might go to special lengths to put a chip into their batteries to correlate to the non-linear discharge curve, I really doubt any of them do.
So the battery discharge curve is typically steep at the start, flat in the middle and steep at the end. What that means is that when the battey is fully charged, you see all the bars, but shortly after you start apply a load, the first bar goes out because the voltage curve is steep as the voltage drops fairly quickly. Then for the majority of the time, the indicator stays on either three or two bars because the battery voltage settles out at a nominal voltage and drops really slowly. Towards the end of the battery discharge cycle, the voltage might still be fairly close to the nominal voltage, but the battery is getting close to being spent. At the end of the cycle, the curve drops off steeply, so that means that you'll go from say two bars to zero quickly. Most of my devices go from two bars to zero in ten minutes or less.
So you shouldn't rely on those bars as anything but a really rough indicator. It's best to do run time tests AND understand the above relationship between number of bars and the run-time so that you'll have a pretty good feeling of how much time you have left on your battery.